Rotational molding, also referred to as rotomolding, is a widely used process to produce hollow articles such as toys, sporting equipment, containers, water tanks, etc. The rotomolding process includes introducing a known amount of plastic in powder form into a hollow, shell-like mold. The mold is rotated about two or more principal axes at relatively low speeds as it is heated so that the plastic enclosed in the mold adheres to and forms a monolithic layer against the mold surface. When the mold rotates in the oven, the oven's metal wall becomes hot and the surface of the powder particles becomes tacky. The particles stick to the mold wall and to each other, thus building up a loose powdery mass against the mold wall. A major portion of the cycle is then taken up in sintering the loose powdery mass until it is a homogeneous melt. The mold rotation continues during the cooling phase so that the plastic retains its desired shape as it solidifies. When the plastic is sufficiently rigid, the cooling and mold rotation is stopped to allow the removal of the plastic product from the mold. At this stage, the cyclic process may be repeated.
In a typical rotational molding process, room temperature plastic is placed in a mold and the whole assembly is heated to a melting temperature for the plastic. Both the mold and the plastic are then cooled back to room temperature. Thus, the rotational molding process requires the mold to both heat and cool the plastic. Generally, the only controls on the process are oven temperature, time in the oven, and rate of cooling. Each of these variables has a major effect on properties of an end product. If the oven time is too short, or the oven temperature is too low, then fusing and consolidation of the plastic will not be complete. This results in low strength, low stiffness, and a lack of toughness in the end product. Conversely, if the plastic is overheated then degradation processes will occur in the plastic, which results in brittleness. In a commercial production environment, the optimum “cooking” time for the plastic in the oven has often been established by trial and error. Moreover, it is noted that once a cooking time has been established for a process, the cooking time is subject to change based upon a change in the ambient room temperature.
It is noted that the disadvantages discussed herein with regards to a rotomolding process are also applicable to other powder processing applications, such as injection molding, blow molding, extrusion, etc.